A possible new development concept for Floating LNG may include a single flowline along the sea floor that splits into dual or more flexible risers. Since the gas co-produces condensate and water, design rules are needed for the splitting of the phases at the riser base manifold. Multiphase flow splitting is much more complex than single-phase flow splitting. The latter is fully determined by the back pressure on each riser, but for multiphase flow the phase split (Liquid-Gas Ratio) into each of the risers may also depend on other factors, such as the flow regime in the riser/flowline, the precise geometric details of the splitting configuration and other parameters (e.g. the momentum flux ratio in the flowline). Ideally the phase volume ratio should be fully equal over the two risers and remain the same as in the flow line. To find the proper design rules for multiphase flow splitting, which ensures equal phase volume split, we have set up a research programme that includes lab experiments and simulations using Computational Fluid Dynamics. The hypothesis is that phase maldistribution can occur if the gas flow rate in the risers is so low that it gives churn flow or hydrodynamic slug flow in the risers, whereas an equal phase volume split is expected if the gas flow rate is sufficiently high to produce annular flow in both risers.

The flow facility at the Shell Technology Centre in Amsterdam transports air and water through a 2", 100 m long flowline, splitting into dual, about 15 m high, risers, having a diameter of 2". The pressure is atmospheric at the riser top. For the splitting configuration, we used a symmetric lay-out, which is a so-called Impacting Tee. We created the splitting curve by systematically changing the opening of the chokes at the top of the risers. At low gas flows a non-symmetric flow split was found, with flip-flopping and hysteresis in the risers. For example, a stagnant liquid flow could develop in one riser, and churn flow in the other riser, with a sudden swap of flow behaviour between the two risers. This maldistribution gradually disappeared as the gas flow rate was increased.

CFD simulations were carried out with the Fluent package using a Volume of Fluid approach for the multiphase flow through the symmetric splitter. As in the experiments, the CFD simulations also give the preference of all flow to be produced through a single riser, with a stagnant liquid column in the other riser. In the experiments this maldistribution disappeared for a sufficiently high gas throughput. For the first flow rate where an equal split is found in the experiments, however, the CFD model still predicts that all of the flow exits through only one of the two risers.

Three-dimensional computational fluid dynamics (CFD) simulations were carried out with ANSYS Fluent 15.0 for the splitting of two-phase, gas-liquid flow from a horizontal flowline to two vertical risers. This piping configuration is representative of what is being envisioned for transport of gas condensate from a subsea production manifold to a floating LNG vessel. The flow conditions that were recently tested in air-water experiments at Shell Technology Centre Amsterdam were simulated. The flow was split using a horizontal impacting tee. The pipe diameter was about 2″ throughout. Simulations were done for a gas (air) flow rate of 60 N m3/hr and a liquid (water) flow rate of 2 m3/hr. In the simulations, the flow split was varied by increasing the pressure at one of the two outlets, leaving the other outlet pressure constant.

The CFD simulations show that if the two outlet pressures are held constant, an unstable flow condition exists that gives transients that finally end up in a state where all flow goes through one riser, whereas a stagnant liquid column is found in the other riser. It was also found that an initially imposed symmetric flow split (i.e., outlet pressures of the two risers being equal) lost its symmetry as the simulation evolved over time, because of the inherent instability and fluctuations in the flow introduced at the impacting tee. This indicates that only two states can exist in this configuration: one in which all flow goes through the first riser and the other in which all flow goes through the second riser.

At the considered flow rate of 60 N m3/hr this behaviour in the CFD simulations is different from the experiments where some finite production through both risers is found. However, for a lower gas throughput (i.e., at 40 and 20 N m3/hr), production through a single riser was measured in the experiments.

Introduction

Gas-condensate fields produce gas and a limited amount of liquid, which consists of condensate and water. Floating LNG (FLNG) is a new way of producing resources from remote fields. A possible concept is using a single flowline that splits into two or more flexible risers. Thus there is vertical upflow in all of the pipe branches downstream of the splitting junction. Due to the limited diameter of flexible risers and limits on the maximum allowed velocity, more than one riser will be needed to transport the produced fluid from the seafloor up to the FLNG vessel. The FLNG vessel is expected to receive fluid produced from multiple gas fields, some of which may be located a significant distance away. Thus having one flowline for each riser can be cost-prohibitive. Being able to achieve stable operation while splitting the flow from a single flowline into multiple risers at the riser base can thus make producing from these distant fields economically viable.

The gas drift velocity in an elongated bubble can be measured as the bubble velocity moving through stagnant liquid in a pipe. In this study, Computational Fluid Dynamics (CFD) is used to numerically simulate the motion of elongated gas bubbles into liquidfilled channels and pipes. The steady, inviscid flow CFD solution agrees with the analytical solution. Furthermore, the CFD solution for viscous flow agrees with new experimental data. Two flow regimes were predicted by the viscous flow simulations: one of constant bubble velocity and another with decreasing bubble velocity over time. A change in flow regime is observed both in terms of the bubble shape and the gas drift velocity. Correlations are derived from the CFD results that describe the time dependent drift velocity as a function of the liquid viscosity.